Crohn's disease patients have more IgG-binding fecal bacteria than controls

In Crohn's disease (CD), chronic gut inflammation leads to loss of mucosal barrier integrity. Subsequent leakage of IgG to the gut could produce an increase of IgG coating of intestinal bacteria. We investigated if there is more IgG coating in patients than in volunteers and whether this is dependent on the host IgG response or on the gut bacteria. Fecal and serum samples were obtained from 23 CD patients and 11 healthy volunteers. Both the in vivo IgG-coated fecal bacteria and in vitro IgG coating after serum addition were measured by flow cytometry and related to disease activity. The bacterial composition in feces was determined using fluorescence in situ hybridization. The IgG-binding capacities of Escherichia coli strains isolated from feces of patients and volunteers were assessed. The results showed that the in vivo IgG-coated fraction of fecal bacteria of patients was slightly larger than that of volunteers but significantly larger after incubation with either autologous or heterologous serum. This was dependent on the bacteria and independent of disease activity or the serum used. The presence of more Enterobacteriaceae and fewer faecalibacteria in patient feces was confirmed. E. coli isolates from patients bound more IgG than isolates from volunteers (P < 0.05) after the addition of autologous serum. Together, these results indicate that CD patients have more IgG-binding gut bacteria than healthy volunteers. We showed that the level of IgG coating depends on the bacteria and not on the serum used. Furthermore, CD patients have a strong specific immune response to their own E. coli bacteria.